The term, black level, may be used to refer i) to the display input signal amplitude required for an image display device to render black in a displayed image (i.e., zero illumination to be provided by the device), or ii) to the sensor output signal amplitude generated by an image sensor when the output signal corresponds to a black region of a scene being captured (i.e., zero illumination received by the sensor). In this specification, the term is used principally to refer to the sensor black level, unless otherwise stated or clear from the context.
Image display devices are typically arranged with a definition that a display input signal of zero amplitude (or a fixed reference voltage, in the case of NTSC) will result in a black displayed image. This is typically also the case with image/video recording devices and digitising systems. Most image sensors, however, have no such definition, so that the sensor output signal is not necessarily of zero amplitude when all light is excluded from the sensor. The amplitude of the sensor output signal may be non-zero to a degree which is affected mainly by the design of the sensor, by manufacturing variations between sensors of the same design, and by temperature of operation. Coupling the sensor output signal to an image display device could therefore result in black regions of a scene being represented by an undefined and most likely non-zero luminance (brightness) in the displayed image (e.g., appearing as grey, instead of black). It is accordingly important for an electronic camera to be able to normalise the sensor output signal; that is, to determine and control the black level so that the display input signal from a camera has zero amplitude (or a fixed reference voltage) to represent black.
The sensing array in an electronic image sensor typically represents the image by producing a complex electrical waveform. Normally, the waveform is sampled by the processing circuitry in synchrony with the pixel clock, to reduce contributions to the signal which do not originate from the scene being captured. The waveform thus typically provides a voltage which varies with the pattern and intensity of light falling on the sensor. Without black level control, the waveform will not have a fixed voltage level to represent black.
The most common technique for providing black level control involves masking a number of pixels in the sensing array such that they have a greatly reduced sensitivity to light. The signal from these pixels is read out of the array to provide a voltage amplitude which is deemed to correspond to the voltage amplitude of a non-illuminated pixel in the active region of the image sensor. Accordingly, the DC voltage level of the waveform from the active region of the sensor is shifted up or down by the voltage amplitude read out from the masked region of the sensor. The voltage amplitude representing black may thereby be adjusted to zero (a fixed, non-zero value may alternatively be chosen).
In one known technique, the information from the masked region may be read out at any suitable stage of the read-out from the array (i.e. before or after all, or part, of the information from the active region). The information from both regions is thereby carried in the same waveform. The black portion of the waveform, from the masked region, is read by an electronic clamp circuit for providing the adjustment to the DC voltage level of the waveform. In this way, the remainder of the waveform is referenced to a fixed black level voltage.
The clamp circuit comprises a driving amplifier and a capacitor, both in series with the video signal processing line, and a clamp switch, which is connected to a black level reference voltage source, which may or may not be ground (i.e. 0V). The clamp switch is arranged to short-circuit the video signal to the reference voltage at times when the video signal passing the clamp switch is the black portion of the waveform. During this clamping step, the capacitor is charged, or discharged, in dependence upon the voltage level of the black portion. When the clamp switch turns off, to remove the short-circuit, the potential difference across the capacitor is considered to provide the DC offset voltage between the actual image sensor output signal black level amplitude and the desired reference amplitude. Operation of the clamp circuit relies on a further amplifier, provided on the signal processing line downstream of the clamp switch, drawing little or no DC current, so that the clamp capacitor is only negligibly discharged before the next clamping step. The clamp circuit continually adjusts the DC voltage level of the sensor output signal, to take account of variations in the sensor black level with time and temperature, at a rate typically in the range from 50 Hz to tens of kilohertz (e.g., if the clamping rate is at line frequency, this is 50 Hz for PAL and 60 Hz for NTSC; if picture frequency, this is 15.625 kHz for PAL and NTSC).
The above technique suffers from a number of problems. Firstly, the clamp circuit operation is reasonably imprecise, as indicated above. Secondly, while the masked region of the sensor array has a greatly reduced sensitivity to light, it is typically not opaque (i.e., it does not have zero sensitivity). Thirdly, it is not appropriate or preferable to control the black level in the above way for some image sensor architectures. In particular, in high-speed video cameras, the image sensor may have multiple output channels for different regions of the sensor array. Since it is desirable to achieve fast read-out of information from the array, if a respective clamp circuit of the above type were provided on each output channel, a significant proportion of the overall read-out time would be spent reading black pixels, when information from the active pixel regions could be read.
In FR-A1-2,844,418, an image embedding process has a first image source and a second image source which are combined in an image output. The first source has a black level generator different to that of the image. The first image black level signal is detected. A switch provides combinations of the two images with the detected black level, and a second combination without the black level.
U.S. Pat. No. 5,070,414 relates to an image reader comprising a line sensor having an array of photoelectric conversion elements, the photoelectric conversion elements being selectively shaded to provide light-shielded pixels for outputting light-shielded signals and effective pixels which are not shaded for outputting effective pixel signals. Analogue-to-digital conversion means convert the light-shielded signals to digital light-shielded signals, which are then held by holding means. Image signal producing means correct a first signal corresponding to the effective pixel signals in accordance with a second signal corresponding to the digital light-shielded signals held by the holding means and generate an image signal.
EP-A1-1,237,353 relates to an image sensor which includes an active pixel area for image capture, one or more black pixel areas disposed in a pre-determined, significant spaced apart distance from the active pixel area, and a light shield to prevent light from illuminating the black pixel areas.
There is a need for an improved or alternative black level control technique. The invention aims to provide such a black level control technique.